The specific requirement which led to the development of this invention was a desire to operate a laser rangefinder at as low a transmitter power as possible, permitting multiple attempts in order to compensate for a very poor S/N ratio. Typical systems now operate with a threshold level, as follows:
1. A voltage threshold is established above the noise level, such that an acceptable false alarm rate is achieved.
2. A signal level is required which will normally be above this threshold level, thus activating "signal present" circuitry upon each crossing of the threshold.
3. Typical threshold to noise voltage ratios vary from 3 to 10, depending on false alarm rates acceptable, and S/N availability. If a low dropout rate (missed signal pulse) is also required, a similar signal to threshold ratio may also be required. Thus operating S/N ratios vary from a minimum of about 3 to very large numbers.